Cellulose suspension and processes for its production

ABSTRACT

The present invention relates to cellulosic particles, a suspension of cellulosic particles and a process for the production of a suspension of cellulosic particles, whereby the cellulosic material is never dried between the dissolution of the cellulose and the disintegration of the suspended cellulose fibers.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to cellulosic particles, a suspension ofcellulosic particles and a process for the production of a suspension ofcellulosic particles, whereby the cellulosic material is never driedbetween the dissolution of the cellulose and the disintegration of thesuspended cellulose fibers.

DESCRIPTION OF RELATED ART

Dry cellulose powders are commercially available in various sizes andare used in a number of different applications, such as for example asauxiliary filter materials, additives and auxiliary materials in foodstuffs and pharmaceutical products, chromatography materials as well asin the form of additives in the building material trade. The greaterpart, which is extracted from pulp, wood or one-year plants, isaccounted for by fibrous cellulose-I-powder. In this respect the lowerlimit of the fiber length of this fibrous cellulose powder is limited to10-20 μm. In the upper range fiber lengths are used in the mm rangewhereby there is already some overlapping with short-cut fibers.

In smaller quantities cellulose-II-powder can also be found wherebyspherical powder can be found in addition to fibrous powder. Thesepowders are mainly made by the precipitation of dissolved cellulose insuitable precipitants. Spherical cellulose powders in the size rangebelow 10 μm can only be made with much more effort and are thusdifficult to find on the market.

For example WO 02/57319 describes the production of cellulose pearlsusing the NMMO-process whereby large amounts of various additives areadded to the cellulose solution prior to forming, such as for exampletitanium dioxide or barium sulphate as well as materials which provokeion exchange. The products obtained can be used as ion exchangers orcatalysts.

The production of a titanium oxide suitable as an ion exchange material,for example for waste water purification, is described in U.S. Pat. No.6,919,029. Particularly high absorption capacities and speeds areattained with this material by means of the fact that the titanium oxidematerial is activated by a special treatment on the surface. Thistitanium oxide material can be described as a “substoichiometrictitanium oxide”. This means that the ratio of the oxygen atoms to thetitanium atoms in the material is smaller than 2. For a more detaileddescription of this surface activation, reference is made to thedescription in U.S. Pat. No. 6,919,029.

Other possibilities for the production of particular functionalizedtitanium oxides can be found in the so-called “doping” of titanium oxidewith iron and sulphur atoms. These compounds display a photocatalyticactivity.

Cellulosic materials in the mm range are likewise being given greaterattention recently. In this respect one can differentiate between therigid crystalline Whiskers (de Souza Lima, M. M. and R. Borsali,Macromolecular Rapid Communications, 2004. 25: p. 771-787) and theflexible MFC (Microfibrillated Cellulose) (Herrick, F. W. et al, Journalof Applied Polymer Science; Applied Polymer Symposium, 1983. 37: p.797-813) and also (Turbak, A. F. F. W. Snyder and K. R. Sandberg,Journal of Applied Polymer Science; Applied Polymer Symposium, 1983. 37:p. 815-827). Both particle types are smaller in the size range of around1 μm and are in the form of suspensions or gels with only a slightcellulose content due to their production process. Production is donelargely via one or several mechanical disintegration steps (ultrasound,homogenizer, etc.) in combination with a strong degradation of thecellulosic starting material via enzymes or strong acids.

Likewise, cryo processes are described in the literature with the helpof liquid nitrogen to release micro-fibrils of cellulosic materials(Chakraborty, A. M. Sain, and M. Kortschot, Holzforschung, 2005. 59: p.102-107).

An alternative method for the production of cellulose nano-fibers is theelectro spinning process (Kulpinski, P., Journal of Applied PolymerScience, 2005. 98 (4): p. 1855-1859) which also demands a great deal ofeffort.

The main field of application for these nano-structured materials iscurrently above all the reinforcement of compound materials (Favier, V.,H. Chanzy and J. Y. Cavaillé, Macromolecules, 1999. 28: p. 6365-6357).

In the literature, films or membranes are described as another specialapplication of the cellulose particles described above. Often thecellulosic materials used in combination with other substances and/orthe production of the films demands a great deal of effort. Examples canbe found in (Fendler, A., et al. Characterization of barrier propertiesof composites of HDPE and purified cellulose fibers. Cellulose, 2007. Inpress. Doi. 10.1007/s10570-007-9136-x), Liu, H. and Y.-L Hsieh,Ultrafine Fibrous Cellulose Membranes from the Electrospinning ofCellulose Acetate. Journal of Polymer Science: part B: Polymer Physics,2002.40:p. 2119-2129) or (Sanchez-Garcia, M.D. E. Gimenez and J. M.Lagaron, Morphology and barrier properties of solvent cast composites ofthermoplastic biopolymers and purified cellulose fibers, CarbohydratePolymers, 2007 in press. doi: 10.1016/j.carbpol.2007.05.041).

SUMMARY OF THE INVENTION

Compared to the known state of the art, the task of the presentinvention was, therefore, to produce cellulose fibrids which fill thegap in the available particle sizes between the nano-materials suspendedin a suspension medium such as for example Whiskers of MFC in the rangesmaller than 1 μm and the conventional dry powders in the range between10 μm to some mm (is this right???) and thereby demonstrate an enhancedeconomic efficiency.

Moreover, the production process should be characterized by its simpleexecution and preferably should work without the strong degradation ofthe cellulosic material via enzymatic or chemical treatment (above allstrong acids) of the starting material since this would involve theelaborate cleaning of the product. Likewise it should be possible toproduce more highly concentrated (˜10% solid substance) suspensions ofthe fibrids produced which can moreover also be readily processed.

It was possible to solve this task via a process for the production of asuspension of cellulosic particles involving the following steps:

Dissolving cellulose to obtain a spinning solution comprising cellulose,

Extruding the solution containing the cellulose,

Precipitating the cellulose wherein cellulose fibers are obtained,

Cutting the precipitated cellulose fibers,

Suspending the cut cellulose fibers and

Disintegrating the suspended cellulose fibers,

-   -   wherein the cellulosic material is never dried between the        dissolving of the cellulose and the disintegrating of the        suspended cellulose fibers.

With this process, cellulosic particles with excellent properties can beobtained with a reduced number of process steps, shorter overallretention times in the process and a reduced need for energy compared tothe present state of the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a film which was made of commercially available drycellulose powder by suspending, spreading the suspension onto an objectslide and finally drying it. The film is very coarse-grained andinhomogeneous as can be readily seen with the naked eye.

FIG. 2 shows a film which is produced by spreading a suspension inaccordance with the invention onto an object plate of glass andsubsequent drying. This film is very homogeneous as can be seen with thenaked eye.

FIG. 3 shows the film of FIG. 2 produced from the suspension inaccordance with the invention under the electron microscope. In thismagnification it can also be seen that the film is very homogeneous.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment, the cellulosic material has a moistness ofabout at least 50% and preferably of about at least 100% and mostpreferably of about at least 150%. For this reason care has to be takenthat the cellulose does not dry out even during further processing,particularly during disintegration, and the process steps in betweeni.e., always has a sufficiently aqueous phase.

Compared to the state of the art, cellulose particles and suspensionswhich contain these particles can be produced with the process accordingto the invention, which have comparable properties and fields ofapplication to the well-known smaller nano-particles but which can beproduced at a more favorable price. An additional economic advantage isthat the suspensions made in accordance with the invention can displaymuch higher cellulose contents, namely up to about 20 weight percentageinstead of only about 2 weight percentage as is the case with thenano-materials. In this way, the packaging and transportation costs dropand likewise the use of the suspensions become more favorably pricedbecause, for example, less suspension medium has to be removed to obtaina film of the cellulose particles.

The production of cellulose fibers on the basis of Lyocell technology iswell known and state of the art. It is fundamental for the production offibrids of fibers in accordance with the invention, that the fibers arenot dried during the process and display a correspondingly high moisturecontent (at least 50%, preferably at least 100% and most preferably atleast 150%).

The best known solvent already being used on a commercial scale inLyocell technology is aqueous N-methyl morpholine-N-oxide (NMMO).However other well-known solvents, in which cellulose is physicallydissolved without chemical derivatization, are both suitable andeconomical for the production of cellulosic fibers in accordance withthe present invention. In particular the dissolution in so-called “ionicliquids” or in other amine oxide containing solvents should be named. Inthe same way the well known viscose processes are suitable andeconomical for the implementation of the present invention in which thecellulose is xanthated in a medium containing caustic soda and therebydissolved, the spinning solution containing cellulose produced in thisway is then extruded and finally precipitated in a precipitation bathand regenerated.

Auxiliary materials can be applied to the moist fibers (e.g., stearicacid-PEG-ester, partly sulphated fatty alcohols and fatty alkylethophosphate) which alleviate further processing, for example, toimprove the grinding ability.

The moist fiber cables are cut by conventional means to a staple lengthof about 2-60 mm. Starting with these initially moist (short-cut)fibers, further processing takes place. When producing the fibers, theproperties of the fibrids obtained at the end can be determined in part.To this end the measurements known to specialists, for example, aresuitable which would lead to an influencing of the fiber titer, fibertenacity, elongation or loop strength if conventional textile fiberswere to be produced. Measurements of this kind are among other thingsthe corresponding selection of pulp with respect to the spinningconditions. While the fiber titer particularly has an effect on theparticle size, the brittleness and fibril tendency can be influenced bythe fiber tenacity. More brittle fibers which fibrillate more easily canbe ground more readily since the duration of grinding is shorter, lessenergy has to be used for grinding and the thermal load of the particleswhen grinding is also reduced.

The fibers are suspended in water or in a suitable other medium wherebythis suspension is homogeneous and should be free-flowing and pumpable.A fiber content in a range of about 4-6% abs.dry, i.e., in relation toabsolutely dry cellulose, has proved to be best suited. The suspendingis performed via suitable mechanical aggregates whereby in addition afibrillation of the fibers can also occur. A fibrillation, i.e., theseparation of individual fibrils from the fiber surface leads to ahigher share of smaller cellulose particles in the suspension due to thefine fractions which are produced in this way. A fine-grain fraction ofthis kind can be either desired or undesired depending on the propertiestargeted in the end product. For this reason, the suspending aggregatemust be selected accordingly.

Several variants are possible for the further process steps:

In the first variant, the fibers are first of all only suspended whereintheir size remains largely unchanged. Finally they are broken down in adisintegration step to the desired final size.

In a second variant, the fibers are already disintegrated duringsuspending and then finally broken down to the final size desired in aconcluding disintegration step. In a third variant, the fibers arealready broken down to the desired final size during suspending.

In a fourth variant it is possible to separate the water from the fibersafter cutting and to disintegrate these moist fibers without anysurrounding liquid in a cutting mill, a high consistency mill or ashredder and then suspending them and breaking them down to the desiredfinal size.

In general it can be said that most devices, which are used in pulpdisintegration during preparation, are also suitable for the productionof a fiber suspension in accordance with this invention. The followingdevices have proved to be suitable for the suspension of the fibers:Ultra Turax with a cutting head, a Jokro mill, a Valley Beater and aRefiner.

In a preferred embodiment of the process in accordance with theinvention, the fibers are disintegrated at the same time as beingsuspended. In this respect suspending devices are suitable of the kindwhich break the fibers down to a length in the range of between about100 μm and about 600 μm at the same time as suspending. This length isparticularly well suited to further disintegration.

In another preferred embodiment of the process in accordance with theinvention, disintegration to a particle size of about 1 to about 5 μmtakes place following suspending. The disintegration is preferably a wetgrinding.

Disintegration is preferably performed with a cellulose content ofbetween about 0.1 and about 5.0 weight percentage in the suspension.With lower cellulose contents, in relation to the quantity of celluloseparticles, the mills required are too big and consequently too high anamount of suspension medium may have to be removed after disintegration.When the cellulose content is too high, the viscosity of the suspensionbecomes too high and too much shearing energy is introduced through thegrinding movement.

Here, as well, it is possible to add additives to support thedisintegration and/or stabilization of the fibrids in the surroundingmedium. Wet grinding takes place in mills with which the desired finalfineness (about 1-5 μm) can be obtained. These are preferably differentagitator bead mills (impeller stirrers, pin stirrers, etc.). Other millssuch as (double) conus mills can also be used but may be lesspreferable.

To achieve the desired particle properties, in particular the finalfineness, it is in general necessary to circulate the suspension in aclosed loop during the wet grinding so that the ground stock can passthrough the mill several times. The fineness can thus be controlled viathe duration of grinding and at the same time it is possible toinfluence the particle size via other parameters. Thus, the size of thegrinding media or the speed of the mill can be adjusted.

Following grinding, a classification of the fibrids is required using awet classifier for example of the type Hosokawa-Alpine Hydroplex when avery narrow distribution is desired.

At the end of grinding, the fibrids are in the suspended medium with alength of about 1-5 μm and a diameter of about 100-500 nm in aconcentration of about 1.1-5 weight percentage. In this respect bothindividual fibrids as well as interconnected fibrid-bonds can be found.The fibrids sediment depending on their size and the type ofstabilization.

To stabilize the suspension, if this is desired, measures can be usedwhich cause the suspension to thicken. This can either be done byremoving a small share of the suspension medium, for example viacentrifuging, evaporation at gentle temperatures or membrane separationprocesses or by adding thickening agents. Suitable thickening agents areknown to the expert, for example commercially availablecarboxymethylcellulose or gylcerin. The use of dispersion agents ortensides as surface-active substances may also stabilize the suspension.This is not, however, a preference because it introduces another classof substance to the suspension in contrast to the polymer orpolymer-like thickening agents.

But even if the cellulose particles in accordance with the inventionsediment in the suspension, no aggregation generally occurs and thefibrid deposition can be easily shaken out again. As a result of thissedimenting in connection with decanting the excess, the concentrationof the fibrids in the suspensions can also be increased. Other methodsto increase the concentration of solids are to centrifuge and evaporatethe liquid phase. In this respect, however, care should be taken thatthe suspensions are not thickened too much otherwise this will lead toirreversible aggregations of the fibrids. The upper limit of the solidcontent in the suspension depends on the fibrid size and, for fibrids,this equals around several μm with 15-20% of dry substance in thesuspension.

The particles can be obtained from suspensions in accordance with theinvention as a result of spray drying. Processes and devices for spraydrying are basically known to experts. It is, however, surprising thatthe suspensions in accordance with the invention can still be sprayedwithout any difficulty even with exceptionally high particle contents.Suspensions with particles which are state of the art with contents of amaximum of 2 weight percentage cannot be readily sprayed because theydisplay high viscosities and a non-Newtonian flow behavior which can beproblematic in the spraying ducts.

In spray drying the individual fibrids do in fact remain intact but theylose their characteristic properties. Spray drying would, however, beinteresting to obtain very fine cellulose powders which are notaccessible via dry grinding.

In accordance with the invention additives which remain on or in thecellulosic particle can be included in the process. In this wayadditional functional properties can be conveyed on the particleswhereby, surprisingly, the good processing properties of the suspensionremain intact. These additives are not removed during the treatment inthe wet state in accordance with the invention such as for examplewashing out, suspending and disintegration, but rather they remain inthe cellulose particles. The additives can be contained in quantities ofbetween about 1 and about 200 weight percentage in relation to thecellulose amount on or in the particle.

These additives can be already added to the cellulosic spinning solutionbefore it is precipitated. They can for example be selected from thegroup consisting of pigments, inorganic substances such as for exampletitanium oxide, such as substoichiometric titanium oxide, bariumsulphate, ion exchangers, polyethylene, polypropylene, polyester,activated carbon, soot, zeolites, polymer superabsorbers andflame-retardant agents.

In the same way additives can be applied to the precipitated cellulosefibers before or after cutting. Suitable devices for this are known tothe expert.

Additives can also be included before, during or after thedisintegration procedure in the suspension in accordance with theinvention. In this case these are mainly distributed on the surface suchas in the outer layers of the cellulosic particles. In this caseadditives can for example be finishings, dyestuffs or cyclodextrins. Inthis way, the processing properties in the process in accordance withthe invention are influenced or, accordingly, the functional propertiesof the particles such as, for example, a higher absorption capacity forcertain substances are achieved.

The subject matter of the present invention is also a suspensioncomprising from about 0.01 to about 20 weight percentage and preferablyfrom about 0.1 to about 10 weight percentage of cellulosic particlesproduced according to the invention whereby the cellulosic particleswere never dried during their production. They form a homogeneous filmwhen drying from the mother suspension. The production of thissuspension can take place according to the process described above inaccordance with the invention. Until now it has not been possible tofind a physical characterization method which records the uniqueproperties of this suspension and the particles contained in it. Thesuspension in accordance with the invention can, however, clearly berecognized by the film formation behavior described here which is uniquefor a suspension of cellulosic particles. Cellulosic particles knownuntil now form homogeneous films only when deliberately applyingelevated temperatures, pressures or additional solvents (see for exampleEndo et al., Polymer Journal (32) 2, 182-185 (2000).

The basic characteristic of the fibrids described is that they areextracted from the cellulose solution and are sufficiently moistthroughout the entire production process. Therefore we are dealing withso-called never-dried particles. Without being bound by any theory, itis believed that the process in accordance with the invention avoids orreduces irreversible hydrogen bridges between the OH groups of thecellulose molecules. For this reason the suspensions described tend toform a homogeneous, dense film when drying up since the OH groups canstill freely arrange themselves.

If one dries the fibrids, introduces them to the suspension again andthen dries them, this results in a film which is in fact not sohomogeneous but clearly more coarse-grained and which has a highertendency to form cracks. The dried up fibrids behave like commerciallyavailable dry cellulose powder when one uses them to form film from thesuspension.

Cellulosic particles with a water content of 80 to 99.9 weightpercentage are also the subject matter of the present invention. Theseare characterized in that they are not dried during their production.They form a homogeneous film when drying up from the mother suspension.

The particles described herein can, as has already been described above,contain a large share of additives. The additives can be included in aquantity of between about 1 and about 200 weight percentage in relationto the cellulose amount in the particles whereby they can be distributedeither in the overall particle or mainly on the surface of the particlesuch as in the outer layers of the particle.

Another object of the invention is the use of cellulosic particles witha water content of about 80 to about 99.9 weight percentage producedusing the process described above to produce homogeneous films.

An advantage of the use of the fibrids according to the invention forfilm formation is, in comparison to the process described above from thestate of the art, its very simple execution. The production of films isdone simply via the gentle drying of the fibrid suspension. In addition,pressure and temperature can also be applied when forming the films. Ahigher temperature accelerates the drying. This is done, for example, byblowing with heated gas, radiant heat or direct contact with heatedsurfaces.

The application of a higher pressure produces, in particular, a denserfilm and is, for example, attained by pressing between flat surfaces orrolls.

In the following, preferred embodiments of the invention are describedusing examples. The invention is, however, not restricted to theseembodiments but rather also encompasses other embodiments which arebased on the same inventive concept.

The size of the particle was determined using a laser diffractionmeasuring apparatus.

EXAMPLE 1

According to what is basically a well known process using NMMO, 6 mmlong Lyocell fibers with an individual fiber titer of 1.3 dtex wereproduced. In this respect the spinning solution was extruded in adry-wet spinning process first of all through an air gap into aprecipitation bath, the gel threads formed were drawn out of theprecipitation bath and then cut following a washing step in a wetcondition. After cutting, the fibers suspended in the water were groundin a Valley Beater (Lorentzen & Wettre). The suspension therebycontained 2.5% cellulose and the grinding duration equalled 150 min. Thesecond grinding took place in a agitator bead mill from Drais Werkemanufacturers with a 1000 ml milling space volume and with zirconiumoxide balls with a diameter of 0.9-1.1 mm for three hours at 2000 rpmand then for another hour at 3000 rpm. The suspension obtained in thisway was finally thickened for 15 hours at 60° C. in the drying cabinetto 7% cellulose. The suspension was viscous and there was no phaseseparation even after a long period of standing. It was possible todilute the suspension again with water without any problems to a lowercellulose content. As a result of thickening (and diluting) thesuspension, no noticeable (irreversible) aggregation of the fibridsoccurred. The length of the fibrids was in the range of 1-8 μm (laserdiffraction, microscopy).

EXAMPLE 2

8 g of cut, moist fibers (cellulose content 30% abs.dry) from example 1(6 mm, 1.3 dtex) was mixed into 60 ml of water using a glass stirrer.This suspension was emptied together with 300 g of zirconium oxide balls(1, 1-1.4 mm diameter) into a stainless steel glass. With an impellerstirrer (IKA RE 166) the suspension was ground for 2 hours at 300 rpm.The grinding balls were separated from the fibrid suspension using asieve. The fibrids were longer than those in example 1. There wereindeed also fibrids with a length of 1 μm, however, the mean value ofthe length was at around 10 μm and fibrids with a length of up to 40 μmcould be found.

EXAMPLE 3

15 g of non-finished, non-dried viscose fibers (1.3 dtex, 38 mm cutlength) from a commercial production line with a cellulose content of35% (abs.dry) were pre-disintegrated and broken into a fibrous state ina laboratory mixer with a Stern knife. 2 g of fibers were dispersed fromthe sample pre-treated in this way by stirring into 80 ml of water usinga glass stirrer. This suspension was mixed with 300 g of zirconium oxideballs (0.9-1.1 mm diameter) in a stainless steel beaker glass andfinally ground for a long time with an impeller stirrer (IKA RE 166) at3000 rpm for 3 hours. The separation of the grinding balls from thefibrid suspension was performed by a sieve. The fibrids obtained were ofa size comparable to those in example 1 with lengths in the range of2-12 μm (laser diffraction).

1.-10. (canceled)
 11. A suspension comprising from about 0.1 to about 20weight percentage, preferably from about 0.1 to about 10 weightpercentage of cellulosic particles, wherein the cellulosic particleswere never dried during their production.
 12. The suspension accordingto claim 11, wherein the cellulosic particles of the suspension comprisea large share of additives.
 13. The suspension according to claim 11,wherein the cellulosic particles of the suspension comprise from about 1to about 200 weight percentage, in relation to the cellulose amount, ofincorporated additives selected from the group consisting of pigments,titanium oxide, barium sulphate, ion-exchangers, polyethylene,polypropylene, polyester, activated carbon, polymer superabsorbers andflame-retardant agents.
 14. Cellulosic particles having a water contentof about 80 to about 99.9 weight percentage wherein the cellulosicparticles are not dried during production.
 15. The particles inaccordance with claim 14, wherein the particles contain a large share ofadditives.
 16. The particles according to claim 14, wherein theparticles comprise from about 1 to about 200 weight percentage, inrelation to the cellulose amount, of incorporated additives, selectedfrom the group consisting of pigments, titanium oxide, barium sulphate,ion-exchangers, polyethylene, polypropylene, polyester, activatedcarbon, polymer superabsorbers and flame-retardant agent.
 17. Theparticles according to claim 13, wherein the titanium oxide issubstoichiometric titanium oxide.
 18. The particles according to claim16, wherein the titanium oxide is substoichiometric titanium oxide.